Electrical battery chargers are well known for the charging of storage batteries. Most chargers are adapted for direct electrical connection to the terminals of a battery to be charged, however, for many applications electrical contacts are a disadvantage. For example, in a miner's cap lamp, the battery pack is conventionally charged by means of an electrical connector fitted to the lamp assembly or battery pack. The connector must have low resistance contacts in order to maintain a consistent, reliable, charging capability. Contamination of the contacts or the corrosion thereof caused by coal dust or other contaminating environment can markedly decrease the efficiency of the electrical contacts with corresponding decrease in the efficiency of battery charging. Moreover, the accessibility of battery contacts or terminals can be a serious problem in explosive environments, as in a coal mine, since short circuiting or tampering with the contacts can produce a spark with possibly disastrous consequences.
Battery chargers have been developed in which electrical contacts are eliminated and electromagnetic coupling employed to convey energy from the charger to a battery. Many such electromagnetically coupled chargers are so-called trickle chargers by which a battery is charged over a relatively long period of time with a small current which continues to flow for so long as the battery is coupled to the charger. These trickle type chargers are usually employed for low energy batteries employed in small battery operated appliances and provide no means for varying the charge or for determining when a fully charged state is reached. This type of charger is shown, for example, in U.S. Pat. Nos. 3,148,552; 3,675,108 and 3,840,795. Another charger is shown in U.S. Pat. No. 3,641,336 in which a battery and charging circuit are contained within a lamp housing and connectable to an external power supply by means of a transformer having a primary winding incorporated into a removable external head and a secondary winding in the lamp housing. A reed switch is connected in series with the primary winding to energize the winding only upon coupling of the two transformer windings for operation.